How Does The Human Nervous System Differ From Those Of Other Animals?

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The nervous arrangement is the function of an animate being's torso that coordinates its behavior and transmits signals between different trunk areas. In vertebrates information technology consists of ii principal parts, called the central nervous arrangement (CNS) and the peripheral nervous arrangement (PNS). The CNS contains the brain and spinal string. The PNS consists mainly of nerves, which are long fibers that connect the CNS to every other part of the body, only also includes other components such as peripheral ganglia, sympathetic and parasympathetic ganglia, and the enteric nervous organization, a semi-independent role of the nervous system whose function is to control the gastrointestinal system.

At the cellular level, the nervous system is defined past the presence of a special type of cell, called the neuron, also known as a "nervus jail cell". Neurons have special properties that permit them to send signals rapidly and precisely to other cells. They transport these signals in the grade of electrochemical waves traveling along thin fibers called axons, which cause chemicals called neurotransmitters to exist released at junctions to other neurons, chosen synapses. A cell that receives a synaptic signal from a neuron (a postsynaptic neuron) may be excited, inhibited, or otherwise modulated. The connections between neurons form neural circuits that can generate very circuitous patterns of dynamical activeness. Along with neurons, the nervous system also contains other specialized cells called glial cells (or just glia), which provide structural and metabolic back up. Recent evidence suggests that glia may besides take a substantial signaling role.

Nervous systems are found in most all multicellular animals, only vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges and microscopic bloblike organisms chosen placozoans and mesozoans. The nervous systems of ctenophores (comb jellies) and cnidarians (e.thou., anemones, hydras, corals and jellyfishes) consist of a diffuse nerve net. All other types of animals, with the exception of echinoderms and a few types of worms, have a nervous system containing a brain, a fundamental cord (or two cords running in parallel), and fretfulness radiating from the encephalon and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans.

At the nearly basic level, the function of the nervous arrangement is to control movement of the organism and to touch the environment (e.k., through pheromones). This is achieved by sending signals from ane prison cell to others, or from i part of the body to others. The output from the nervous organization derives from signals that travel to muscle cells, causing muscles to be activated, and from signals that travel to endocrine cells, causing hormones to be released into the bloodstream or other internal fluids. The input to the nervous system derives from sensory cells of widely varying types, which transmute concrete modalities such as light and sound into neural activity. Internally, the nervous organization contains complex webs of connections between nerve cells that let it to generate patterns of activity that depend only partly on sensory input. The nervous system is besides capable of storing data over fourth dimension, by dynamically modifying the strength of connections between neurons, besides as other mechanisms.

i Structure
- 1.ane Cells
  - 1.1.1 Neurons
  - 1.i.ii Glial cells
two Anatomy in vertebrates
three Comparative anatomy and evolution
- 3.one Neural precursors in sponges
- 3.two Radiata
- iii.3 Bilateria
  - iii.iii.1 Annelids
  - iii.3.2 Ecdysozoa
    - iii.3.2.one Nematodes
    - 3.iii.two.2 Arthropods
- iii.4 "Identified" neurons
four Part
- iv.ane Neurons and synapses
- four.2 Neural circuits and systems
- iv.3 Reflexes and other stimulus-response circuits
- iv.iv Intrinsic design generation
5 References

Structure

The nervous organisation derives its name from nerves, which are cylindrical bundles of fibers that emanate from the brain and central cord, and branch repeatedly to innervate every part of the body. Fretfulness are big enough to have been recognized by the ancient Egyptians, Greeks, and Romans (Finger, 2001, chapter 1), but their internal structure was not understood until it became possible to examine them using a microscope. A microscopic examination shows that fretfulness consist primarily of the axons of neurons, along with a variety of membranes that wrap around them. The neurons that requite rising to nerves do not generally lie within the fretfulness themselves — their cell bodies reside within the brain, central string, or peripheral ganglia.

All animals more derived than sponges have nervous systems. However, even sponges, unicellular animals, and not-animals such as slime molds have prison cell-to-prison cell signalling mechanisms that are precursors to those of neurons (Sakarya et al., 2007). In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a lengthened network of isolated cells. In bilaterian animals, which make upwardly the peachy bulk of existing species, the nervous system has a common construction that originated early on in the Cambrian menstruation, over 500 million years ago.

Cells

The nervous system contains two primary categories or types of cells: neurons and glial cells.

Neurons

The nervous organization is divers by the presence of a special type of cell, the neuron (sometimes called "neurone" or "nerve cell"). Neurons can exist distinguished from other cells in a number of ways, but their almost primal property is that they communicate with other cells via synapses, which are junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an axon, a protoplasmic protrusion that tin can extend to distant parts of the trunk and brand thousands of synaptic contacts. Axons frequently travel through the trunk in bundles chosen nerves (in the PNS) or tracts (in the CNS).

Even in the nervous system of a single species such as humans, hundreds of different types of neurons be, with a wide diverseness of morphologies and functions. These include sensory neurons that transmute physical stimuli such equally light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands. In many species, though, the majority of neurons receive all of their input from other neurons and transport their output to other neurons.

Glial cells

Glial cells (named from the Greek word for "glue") are not-neuronal cells that provide support and nutrition, maintain homeostasis, grade myelin, and participate in betoken transmission in the nervous organization (Allen, 2009). In the man brain, it is currently estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different encephalon areas (Azevedo et al., 2009). Among the near important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy pathogens and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets. A very important prepare of glial cell (oligodendrocytes in the vertebrate CNS, and Schwann cells in the PNS) generate layers of a fatty substance called myelin that wrap around axons and provide electrical insulation that allows them to transmit signals much more rapidly and efficiently.

Beefcake in vertebrates

Effigy 1: Major divisions of the vertebrate nervous system.

The nervous arrangement of vertebrate animals is divided into two parts called the fundamental nervous organization (CNS) and peripheral nervous system (PNS).

The CNS is the largest function, and includes the brain and spinal string. The CNS is enclosed and protected by meninges, a 3-layered arrangement of membranes, including a tough, leathery outer layer called the dura mater. The brain is also protected by the skull, and the spinal cord past the vertebral basic. Blood vessels that enter the CNS are surrounded by cells that course a tight chemical seal called the blood-brain barrier, preventing many types of chemicals present in the trunk from gaining entry to the CNS.

The peripheral nervous arrangement (PNS) is a commonage term for the nervous system structures that exercise not lie within the CNS. The big majority of the axon bundles called nerves are considered to belong to the PNS, fifty-fifty when the cell bodies of the neurons to which they vest reside within the brain or spinal string. The PNS is divided into "somatic" and "visceral" parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglion of the spinal cord. The visceral office, also known every bit the autonomic nervous system, contains neurons that innervate the internal organs, claret vessels, and glands. The autonomic nervous system itself consists of ii parts: the sympathetic nervous organization and the parasympathetic nervous system. Some authors besides include sensory neurons whose jail cell bodies lie in the periphery (for senses such equally hearing) as office of the PNS; others, all the same, omit them (Hubbard, 1974, p. vii).

The vertebrate nervous system tin can also exist divided into areas called grey matter ("grayness matter" in British spelling) and white affair. Gray matter (which is only gray in preserved tissue, and is better described as pink or lite brown in living tissue) contains a high proportion of prison cell bodies of neurons. White thing is composed mainly of myelin-coated axons, and takes its colour from the myelin. White thing includes all of the body's nerves, and much of the interior of the brain and spinal string. Grayness matter is plant in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. In that location is an anatomical convention that a cluster of neurons in the brain is chosen a "nucleus", whereas a cluster of neurons in the periphery is called a "ganglion". There are, however, a few exceptions to this rule, notably the part of the brain chosen the basal ganglia.

Comparative anatomy and evolution

Neural precursors in sponges

Sponges have no cells continued to each other by synaptic junctions, that is, no neurons, and therefore no nervous arrangement. They do, however, have homologs of many genes that play key roles in synaptic function in other animals. Recent studies accept shown that sponge cells express a grouping of proteins that cluster together to form a construction resembling a postsynaptic density (the indicate-receiving part of a synapse) (Sakarya, 2007). Yet, the function of that structure is currently unclear. Although sponge cells do not evidence synaptic transmission, they practice communicate with each other via calcium waves and other impulses, which mediate some unproblematic deportment such as whole-body contraction (Jacobs et al., 2007).

Radiata

Jellyfish, comb jellies, and related animals have diffuse nervus nets rather than a central nervous system. In nearly jellyfish the nerve net is spread more or less evenly across the trunk; in comb jellies it is concentrated nigh the mouth. The nerve nets consist of sensory neurons, which pick upward chemical, tactile, and visual signals; motor neurons, which can actuate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, ship signals to groups of motor neurons. In some cases groups of intermediate neurons are amassed into discrete ganglia (Ruppert et al., 2004).

The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only take two primordial cell layers, the endoderm and ectoderm. Neurons are generated from a special set up of ectodermal precursor cells, which also serve every bit precursors for every other ectodermal cell type (Sanes et al., 2006).

Bilateria

Figure 2: Nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "encephalon" at the front. (Note: this drawing shows the nervus string on the dorsal side of the body, but as the article explains, in protostomes it by and large lies on the ventral side.)

The vast bulk of existing animals are bilaterians, pregnant animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to take descended from a common wormlike ancestor that appeared during the Cambrian period, 550–600 1000000 years ago (Balavoine, 2003). The fundamental bilaterian trunk form is a tube with a hollow gut cavity running from rima oris to anus, and a nerve cord (or two parallel nervus cords), with an enlargement (a "ganglion") for each trunk segment, with an especially large ganglion at the front, called the "encephalon". It has not been definitively established whether the generic class of the bilaterian central nervous system is inherited from the and so-called "Urbilaterian" — the concluding common antecedent of all existing bilaterians — or whether separate lines have evolved similar structures in parallel (Northcutt, 2012). On one manus, the presence of a shared set of genetic markers, as well as a tripartite brain structure shared by widely separated species (Hirth, 2010), suggest common derivation; on the other hand, the fact that some modern types of bilaterians (such as echinoderms) lack a cardinal nerve cord, while many lack recognizably tripartite brains, suggest that this might accept been the archaic state (Northcutt, 2012).

Vertebrates, annelids, crustaceans, and insects all show the segmented bilaterian body plan at the level of the nervous system. In mammals, the spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is circuitous, but on the trunk it gives rise to a series of narrow bands. The superlative three segments belong to the brain, giving ascent to the forebrain, midbrain, and hindbrain (Ghysen, 2003).

Bilaterians can exist divided, based on events that occur very early in embryonic evolution, into two groups (superphyla) called protostomes and deuterostomes (Erwin et al., 2002). Deuterostomes include vertebrates likewise equally echinoderms, hemichordates (mainly acorn worms), and Xenoturbellidans (Bourlat et al., 2006). Protostomes, the more diverse grouping, include arthropods, molluscs, and numerous types of worms. There is a basic difference between the two groups in the placement of the nervous system inside the torso: protostomes possess a nerve string on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve string is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that bear witness dorsal-to-ventral gradients. Most anatomists at present consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed past Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline (Lichtneckert and Reichert, 2005).

Annelids

Figure 3: Earthworm nervous organization. Top: side view of the front of the worm. Bottom: nervous system in isolation, viewed from above

Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the almost straightforward style. As an case, earthworms accept dual nerve cords running forth the length of the trunk and merging at the tail and the rima oris. These nerve cords are continued to each other past transverse nerves resembling the rungs of a ladder. These transverse nerves help coordinate movement of the ii sides of the animal. Two ganglia at the head end function as a uncomplicated brain. Photoreceptors in the brute'due south eyespots provide sensory data on light and dark (Adey, WR).

Ecdysozoa

Ecdysozoa are animals that shed their cuticle. These include nematodoes and arthropods.

Nematodes

The nervous organization of one particular blazon of nematode, the tiny roundworm Caenorhabditis elegans, has been mapped out down to the synaptic level. This has been possible considering in this species, every private worm (ignoring mutations and sex differences) has an identical ready of neurons, with the same locations and chemical features, and the same connections to other cells. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are mapped. The nervous system of C. elegans is sexually dimorphic; the nervous systems of the two sexes, males and hermaphrodites, accept unlike numbers of neurons and groups of neurons that perform sex-specific functions. Males take exactly 383 neurons, while hermaphrodites have exactly 302 neurons (Hobert, 2005), an unusual characteristic called eutely.

Arthropods

Arthropods, such as insects and crustaceans, have a nervous organization fabricated up of a serial of ganglia, connected past a pair of ventral nervus cords running along the length of the abdomen (Chapman, 1998). Most body segments have one ganglion on each side, only some are fused to form the brain and other big ganglia. The head segment contains the brain, besides known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands and certain muscles. Many arthropods take well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory data from these organs is processed past the brain.

In arthropods, most neurons have prison cell bodies that are positioned at the edge of the brain and are electrically passive — the cell bodies serve only to provide metabolic support and practise not participate in signalling. A protoplasmic fiber, called the master neurite, runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, nearly parts of the insect encephalon have passive cell bodies arranged around the periphery, while the neural bespeak processing takes place in a tangle of protoplasmic fibers chosen "neuropil", in the interior (Chapman, 1998). There are, however, important exceptions to this dominion, including the mushroom bodies, which play a key role in learning and memory.

"Identified" neurons

A neuron is called identified if it has backdrop that distinguish it from every other neuron in the same animal — such as location, neurotransmitter, cistron expression pattern, and connectivity — and if every individual organism belonging to the same species has one and only one neuron with the same set of properties (Hoyle and Wiersma, 1977). In vertebrate nervous systems very few neurons are "identified" in this sense — in humans, at that place are believed to be none — merely in simpler nervous systems, some or all neurons may be thus unique. Equally mentioned above, in the roundworm Caenorhabditis Elegans every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm.

The brains of many molluscs and insects also contain substantial numbers of identified neurons (Hoyle and Wiersma, 1977). In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish (Stein, 1999). Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and i on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same encephalon level then traveling downwardly through the spinal string, making numerous connections every bit it goes. The synapses generated past a Mauthner prison cell are and then powerful that a unmarried activity potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape, then straightens, thereby propelling itself quickly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure moving ridge impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish — in that location are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all past itself, in the context of ordinary behavior other types of cells ordinarily contribute to shaping the amplitude and direction of the response.

Mauthner cells have been described as "command neurons". A command neuron is a special type of identified neuron, defined every bit a neuron that is capable of driving a specific beliefs individually (Stein, 1999, p. 112). Such neurons appear most commonly in the fast escape systems of various species — the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, go controversial, because of studies showing that some neurons that initially appeared to fit the clarification were really merely capable of evoking a response in a limited fix of circumstances (Simmons and Young, 1999).

Office

The ultimate part of the nervous system is to control the trunk, especially its movement in the environment. Information technology does this past extracting information from the environment using sensory receptors, sending signals that encode this data into the central nervous organisation, processing the data to make up one's mind an appropriate response, and sending output signals to muscles or glands to actuate the response. The development of a complex nervous system has made information technology possible for various creature species to take advanced perceptual capabilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes information technology possible to take language, abstract representation of concepts, transmission of civilization, and many other features of homo society that would not exist without the human brain.

At the nearly bones level, the nervous system sends signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can transport signals to other cells. One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In dissimilarity to this "broadcast" mode of signaling, the nervous system provides "signal-to-bespeak" signals — neurons projection their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is as well much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second.

Neurons and synapses

Figure 4: Major elements in synaptic transmission. An electrochemical wave called an action potential travels along the axon of a neuron. When the wave reaches a synapse, it provokes release of neurotransmitter molecules, which bind to chemical receptor molecules located in the membrane of the target cell.

About neurons ship signals via their axons, although some types are capable of emitting signals from their dendrites. In fact, some types of neurons such as the amacrine cells of the retina have no axon, and communicate just via their dendrites. Neural signals propagate along an axon in the class of electrochemical waves called action potentials, which emit prison cell-to-cell signals at points of contact called "synapses".

Synapses may exist electrical or chemical. Electric synapses pass ions directly between neurons (Hormuzdi et al., 2004), but chemic synapses are much more common, and much more than diverse in role. At a chemical synapse, the prison cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic regions of contact are full of molecular machinery that carries out the signalling process. The presynaptic surface area contains large numbers of tiny spherical vessels called synaptic vesicles, packed with neurotransmitter chemicals. When calcium enters the presynaptic last through voltage-gated calcium channels, an arrays of molecules embedded in the membrane are activated, and cause the contents of some vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft. The neurotransmitter then binds to chemical receptors embedded in the postsynaptic membrane, causing them to enter an activated country. Depending on the blazon of receptor, the effect on the postsynaptic cell may exist excitatory, inhibitory, or modulatory in more complex ways. For example, release of the neurotransmitter acetylcholine at a synaptic contact betwixt a motor neuron and a muscle cell depolarizes the musculus jail cell and starts a serial of events, which results in a contraction of the muscle cell. The entire synaptic transmission process takes but a fraction of a millisecond, although the effects on the postsynaptic prison cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace).

There are literally hundreds of different types of synapses, even within a single species. In fact, there are over a hundred known neurotransmitter chemicals, and many of them activate multiple types of receptors. Many synapses use more than one neurotransmitter — a mutual system is for a synapse to utilise 1 fast-acting small-molecule neurotransmitter such every bit glutamate or GABA, along with one or more than peptide neurotransmitters that play slower-acting modulatory roles. Neuroscientists generally divide receptors into two broad groups: ligand-gated ion channels and Yard-protein coupled receptors (GPCRs) that rely on second messenger signaling. When a ligand-gated ion channel is activated, information technology opens a channel that allow specific types of ions to flow across the membrane. Depending on the type of ion, the effect on the target prison cell may be excitatory or inhibitory by bringing the membrane potential closer or farther from threshold for triggering an activity potential. When a GPCR is activated, it starts a cascade of molecular interactions inside the target prison cell, which may ultimately produce a wide diversity of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription.

According to Dale'due south principle, which has simply a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses (Strata and Harvey, 1999). This does non hateful, though, that a neuron exerts the same consequence on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on the receptors that it activates. Because different targets tin (and often exercise) use dissimilar types of receptors, information technology is possible for a neuron to have excitatory furnishings on 1 set of target cells, inhibitory furnishings on others, and complex modulatory effects on others yet. Nevertheless, information technology happens that the two virtually widely used neurotransmitters, glutamate and gamma-Aminobutyric acid (GABA), each have largely consistent furnishings. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, simply all of them are inhibitory. (At that place are a few exceptional situations in which GABA has been found to have excitatory effects, mainly during early development. For a review come across Marty and Llano, 2005.) Because of this consistency, glutamatergic cells are oft referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking this is an corruption of terminology — information technology is the receptors that are excitatory and inhibitory, not the neurons — but it is commonly seen fifty-fifty in scholarly publications.

Ane very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic strength. The best-understood form of neural retentivity is a process called long-term potentiation (abbreviated LTP), which operates at synapses that apply the neurotransmitter glutamate acting on a special blazon of receptor known as the NMDA receptor (Cooke and Bliss, 2006). The NMDA receptor has an "associative" holding: if the two cells involved in the synapse are both activated at approximately the same fourth dimension, a channel opens that permits calcium to flow into the target cell (Elation and Collingridge, 1993). The calcium entry initiates a 2d messenger cascade that ultimately leads to an increase in the number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. This change in force can concluding for weeks or longer. Since the discovery of LTP in 1973, many other types of synaptic memory traces have been found, involving increases or decreases in synaptic forcefulness that are induced past varying atmospheric condition, and last for variable periods of fourth dimension (Cooke and Bliss, 2006). Reward learning, for case, depends on a variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses dopamine as neurotransmitter (Kauer and Malenka, 2007). All these forms of synaptic modifiability, taken collectively, give rise to neural plasticity, that is, to a capability for the nervous organization to suit itself to variations in the environment.

Neural circuits and systems

The basic neuronal part of sending signals to other cells includes a adequacy for neurons to exchange signals with each other. Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including characteristic detection, pattern generation, and timing (Dayan and Abbott, 2005). In fact, information technology is difficult to assign limits to the types of information processing that can be carried out by neural networks: Warren McCulloch and Walter Pitts proved in 1943 that fifty-fifty artificial neural networks formed from a greatly simplified mathematical brainchild of a neuron are capable of universal computation. Given that individual neurons can generate circuitous temporal patterns of activity independently, the range of capabilities possible for even minor groups of neurons are across current understanding.

Figure five: Illustration of pain pathway, from René Descartes's Treatise of Human

Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator (Sherrington, 1906). In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving ascent eventually to activation of motor neurons and thereby to muscle contraction, i.eastward., to overt responses. Descartes believed that all of the behaviors of animals, and about of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he likewise believed that higher cerebral functions such equally language were not capable of being explained mechanistically. Charles Sherrington, in his influential 1906 book The Integrative Activity of the Nervous System, developed the concept of stimulus-response mechanisms in much more detail, and Behaviorism, the school of thought that dominated Psychology through the heart of the 20th century, attempted to explain every attribute of human beliefs in stimulus-response terms (Baum, 2005).

Even so, experimental studies of electrophysiology, beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for generating patterns of activeness intrinsically, without requiring an external stimulus (Piccolino, 2002). Neurons were institute to be capable of producing regular sequences of activity potentials, or sequences of bursts, even in consummate isolation. When intrinsically agile neurons are connected to each other in complex circuits, the possibilities for generating intricate temporal patterns become far more extensive. A modern formulation views the office of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns — both types of activity collaborate with each other to generate the full repertoire of behavior.

Reflexes and other stimulus-response circuits

Figure 6: Simplified schema of bones nervous system office: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs that results in signals sent back to the spinal cord so out to motor neurons

The simplest type of neural circuit is a reflex arc, which begins with a sensory input and ends with a motor output, passing through a sequence of neurons in betwixt. For example, consider the "withdrawal reflex" causing the paw to wiggle back after a hot stove is touched. The circuit begins with sensory receptors in the peel that are activated by harmful levels of heat: a special type of molecular construction embedded in the membrane causes heat to change the electrical field across the membrane. If the change in electrical potential is big enough, it evokes an activity potential, which is transmitted forth the axon of the receptor cell, into the spinal string. There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into the brain. One target is a gear up of spinal interneurons that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate activeness potentials, which travel down their axons to the point where they brand excitatory synaptic contacts with musculus cells. The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm abroad.

In reality, this straightforward schema is subject area to numerous complications. Although for the simplest reflexes there are short neural paths from sensory neuron to motor neuron, in that location are besides other nearby neurons that participate in the excursion and modulate the response. Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex.

Although the simplest reflexes may be mediated past circuits lying entirely within the spinal cord, more circuitous responses rely on signal processing in the brain. Consider, for example, what happens when an object in the periphery of the visual field moves, and a person looks toward information technology. The initial sensory response, in the retina of the eye, and the terminal motor response, in the oculomotor nuclei of the brain stem, are not all that unlike from those in a simple reflex, merely the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus, cognitive cortex, basal ganglia, superior colliculus, cerebellum, and several brainstem nuclei. These areas perform bespeak-processing functions that include feature detection, perceptual analysis, memory recall, controlling, and motor planning.

Feature detection is the ability to extract biologically relevant information from combinations of sensory signals. In the visual organisation, for example, sensory receptors in the retina of the heart are simply individually capable of detecting "dots of light" in the outside world. 2d-level visual neurons receive input from groups of main receptors, college-level neurons receive input from groups of second-level neurons, then on, forming a hierarchy of processing stages. At each stage, important data is extracted from the signal ensemble and unimportant information is discarded. By the end of the procedure, input signals representing "dots of light" have been transformed into a neural representation of objects in the surrounding world and their backdrop. The well-nigh sophisticated sensory processing occurs inside the brain, only complex characteristic extraction too takes place in the spinal string and in peripheral sensory organs such as the retina.

Intrinsic design generation

Although stimulus-response mechanisms are the easiest to understand, the nervous arrangement is besides capable of controlling the torso in ways that do not require an external stimulus, past means of internally generated patterns of activeness. Because of the variety of voltage-sensitive ion channels that tin exist embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of activity potentials, or rhythmic alternations betwixt high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and fifty-fifty chaos. A network of neurons that uses its internal structure to generate spatiotemporally structured output, without requiring a correspondingly structured stimulus, is called a central pattern generator.

Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is cyclic rhythmicity — that is, rhythmicity with a period of approximately 24 hours. All animals that accept been studied show circadian fluctuations in neural action, which command cyclic alternations in behavior such as the slumber-wake cycle. Experimental studies dating from the 1990s take shown that cyclic rhythms are generated past a "genetic clock" consisting of a special set up of genes whose expression level rises and falls over the course of the day. Animals every bit diverse equally insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-twenty-four hour period cues are available. The clock genes are expressed in many parts of the nervous system likewise as many peripheral organs, but in mammals all of these "tissue clocks" are kept in synchrony by signals that emanate from a main timekeeper in a tiny part of the brain called the suprachiasmatic nucleus.

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Source: http://www.scholarpedia.org/article/Nervous_system

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